Several species of monkeys were inoculated with JC, BK or SV-40-PML strains of papovaviruses. One of these isolates come from a human patient with Progressive Multifocal Leukoencephalopathy (PML). The inoculated animals have been followed for two years. PML has not developed in any of the animals to date. However, lesions in the CNS have been noted in two of the inoculated monkeys. These lesions are being evaluated at the present time for their possible relevance to human disease.